The present invention relates to a method and apparatus for handling a submersible. More particularly, this invention relates to a system, based on a floating mother ship, for launching, retrieving, and servicing a submersible.
The rapid depletion of a broad spectrum of raw materials and particularly a worldwide shortage of petroleum has induced exploration for these natural resources to be intensified and extended to most land areas of the earth. Formerly uncharted regions have been subjected to intense scrutiny from various land based systems, aircraft, and space vehicles. However, despite a major effort to locate and develop new sources of raw materials, the supply has continued to fall considerably short of demand. The economic consequence, of course, has been a substantial increase in the cost of raw materials.
As the price of these raw materials has increased, the exploration and exploitation of mineral resources from submerged land areas has become commercially more attractive. As an example, nodules rich in various minerals are now being mined from the floor of the ocean. Moreover, oil wells are being drilled in increasingly deep regions of continental shelves adjacent littoral states.
In most cases, once a potentially productive field is located, a well is drilled from a floating or semisubmersible vessel or a fixed platform. For examples of fixed platform structures, reference may be had to U.S. Pat. Nos. 3,585,801 and 3,668,876, to Koehler issued June 22, 1971, and June 13, 1972, respectively.
With the foregoing drilling and subsequent production activity, a problem arose in connection with an economical means for transporting the crude petroleum and gas from a drilling site to a shore collection location or an offshore transfer station. Frequently, the most economic means for accomplishing transfer has been to construct submerged pipelines between the producing and collection locations. In this connection, U.S. Pat. Nos. to Hauber et al 3,280,571 issued Oct. 25, 1966; Lawrence 3,390,532 issued July 2, 1968; Smith 3,566,609 issued Mar. 2, 1971; Nolan 3,645,105 issued Feb. 29, 1972; and Jones et al 3,668,878 issued June 13, 1972, disclose highly effective methods and apparatus for laying pipeline upon the bed of a body of water.
Shortly following the discovery that pipelines could be economically laid on a water bed significant problems were encountered such as; shifting of the line due to current, pipeline corrosion, and pipeline damage caused by anchors, fishnets and similar equipment. In order to obviate or minimize the above-noted difficulties, it has become an industry practice to bury or entrench the submerged pipeline structures beneath the water bed.
In this connection a number of burying techniques have been successfully utilized such as, for example, those disclosed in U.S. Pat. Nos. Tittle 3,338,059 issued Aug. 29, 1967; Perot 3,751,927 issued Aug. 14, 1973; and Good et al 3,786,642 issued Jan. 22, 1974.
Once the previously noted pipe laying operation is complete, economic and ecological considerations dictate a thorough inspection of the line for kinks, cracks, loss of weight coating, etc. In a similar vein, the line must again be inspected following the burying operation.
Initially, in shallow water regions, this inspection operation was performed by a diver walking along the line and visually inspecting it for defects. However, as lines extended into deeper waters it rapidly became apparent that other techniques would be required. In this connection, over the past decade or so, untethered, deep-operating, self-propelled craft, sometimes referred to as submersibles, have been effectively utilized in inspection work.
Submersibles presently inn use in typically two- or three-man crafts which are normally ferried on the deck of a mother ship to the line to be inspected. The submersible systems are powered by rechargeable direct current cells carried by the craft. Once an inspection is completed or the power cells are spent, the submersible returns to the surface and is retrieved by the mother vessel for storage and/or servicing.
Further in the above connection, submersibles are typically lowered into the water by a suitable A-frame crane or davit located on the deck of the mother ship. The position of the submersible relative to the mother ship and the A-frame crane has been previously steadied by a plurality of lead lines connected to the submersible and manipulated by deck hands operating from the deck of the mother vessel. Once the submersible was in the water, divers released the crane and lead lines and the submersible could begin a diving operation.
When inspection of the pipeline system was completed or the power cells became depleted, the submersible would return to the surface to be retrieved in a manner similar to that employed in the launching of the craft. More particularly, divers would attach lead lines and a main hoisting line to the submersible and the submersible would be hoisted from the water while deck hands again engaged in steadying the vessel. Once deposited and secured on the mother vessel, the submersible's power cells could be removed and replaced or recharged.
While the above general arrangement and procedure may have been relatively satisfactory in the past, room for significant improvement remains.
In this regard, when seas are rough, it may be dangerous to have divers in the water, particularly in the vicinity of the mother vessel and the submersible. A diver may become tangled in the lines used to hoist or steady the submersible. Furthermore, a diver could be swept by waves into contact with the screws or rudders of the mother ship.
Rouch seas may also cause problems once a submersible is suspended from an A-frame crane since it may be impossible to exert sufficient control over perpendicular or swinging movement of the submersible. The submersible may also tend to spin in a generally horizontal plane at the end of the line used to suspend the submersible from the crane. Movement of this type may render the submersible vulnerable to damage due to impacting with the crane or mother ship.
This general phenomenon of pendular and/or spinning motion of the submersible may be controllable by hand lines in quiet waters. In waters as rough as the North Sea, however, such an arrangement may be impractical since it might simply require too many men. Indeed, stabilization of the submersible by means of lead lines may not be possible with a normal crew, particularly considering the fact that footing abroad a pitching and rolling vessel may be difficult to maintain.
Further, in rough seas, a reliable, quick and safe method and apparatus must be available to connect a line to the floating submersible. If divers are employed to connect the various lines to the submersible, the operation may not only be unsafe, as previously noted, but may also be slow and/or unreliable in that opportunities to effect the connection may be missed. Similarly, if independent capture lines are used, the connection may be undesirable since it may be extremely difficult to effect a proper engagement of the capture lines by the submersible.
A further problem with the prior submersible handling arrangements and procedures resides in the manner in which the power cells which provide energy to the submersible are recharged. At least three different techniques have been previously known.
As indicated above, the power cells are commonly disposed in long cylindrical pods attached to the hull of the submersible. Once the cells are spent and the submersible has been retrieved by the mother vessel, the power cells may be recharged in place within the pods. A considerable length of time, however, may be consumed in the course of this recharging operation. To alleviate significant time gaps in a working cycle, a second submersible is required so that one submersible can be in service while the power cells of the other are being recharged. This requirement of twin submersibles obviously causes a considerable increase in the overall overhead cost of the inspection operation. It also may complicate the handling equipment on board the mother ship since two submersibles must be accommodated.
Alternatively, the pods containing the power cells may be opened, the power cells removed, and new or recharged cells installed. Due to the cramped working quarters and the nature of this task, such replacement of power cells can be a quite laborious and time-consuming process. Indeed, using this particular technique, a submersible carrying a compliment of fifty power cells may be out of operation for as long as eight hours. Thus, if only a single submersible is used, the efficiency of the entire operation is again sufficiently impaired.
A third technique entails replacing the power cells in an automated process while the submersible is submerged. This operation requires the use of a habitat or other similar submerged base. Commonly a package of fresh power cells is disposed on the exterior of the habitat or base. The submersible docks with the habitat and a ram removes the spent package of power cells and slides the fresh package into place. This technique may be subject to a number of problems. For instance, conducting the exchange of power cells in a submerged environment renders the operation vulnerable to a number of accidents, any one of which could leave the submersible crippled and the occupants trapped beneath the surface of the water. Should the ram fail after having removed the spent power cells and prior to installing the fresh power cells, the submersible would be without power and perhaps unable to surface or maneuver. Also, as indicated, this system requires the use of a habitat or other similar submerged base for the removal and replacement operation.
A further problem somewhat related to that of replacing the power cells resides in the exposure of the submersible and the workmen replacing the power cells and otherwise servicing the vessel to ambient elements. In this connection, weather on the North Sea is often highly inclement. In many of the arrangements and procedures for replacing the power cells and servicing the submersible, the vessel may be simply secured to the deck of the mother vessel. Thus, men removing the power cells and servicing the submersible may be subjected to the winds, rain, and cold characteristic of the region. This, of course, hampers the overall operation and may render the men more vulnerable to injury.
The problems suggested in the preceding are among many which may tend to reduce the effectiveness of arrangements and procedures of the prior art employed to handle and service submersibles. Other noteworthy problems may also exist; however, those presented in the discussion above should be sufficient to demonstrate that the methods and apparatus for launching, retrieving, and servicing submersibles presented in the prior art have not been altogether satisfactory.